Resorcinol compounds have been widely used in various applications including rubber compounding and fabric dipping technologies. In rubber compound formulations, resorcinol resins have been widely used as methylene acceptors. Although the resorcinol resins generally provide sufficient adhesion properties, it is still desirable to improve the dynamic properties, such as storage modulus and tangent delta, of the rubber compounds by using novel resorcinol compounds.
The dipping technology has been extensively used throughout the rubber and tire industries to enhance the adhesion of rubber reinforcing materials such as fibers, filaments, fabrics or cords of polyesters (such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN)), polyamides (such as nylons and aramids), carbon or polybenzoxazole (PBO) to natural as well as synthetic rubbers. For improving the adhesion of rubbers to fibers of polyesters or polyamides, numerous modifications have been made in the dipping formulations. Among these modifications, the addition of blocked aromatic diisocyanates appeared to enhance the adhesion of PET to rubbers. In general, blocked diisocyanates, particularly the caprolactam- and phenol-blocked diisocyanate have been widely used by the rubber and tire industries. Some common examples of caprolactam- and phenol-blocked diisocyanates are caprolactam- and phenol-blocked 4,4′-diphenylmethane diisocyanate (4,4′-MDI).
The use of phenol-blocked diisocyanates such as phenol-blocked 4,4′-MDI have been restricted in dipping formulations, possibly due to their high unblocking temperatures. Further, under the process temperature of the fabric-treating technology, which generally is between 150° C. and 240° C., the unblocking reaction produces phenol from the phenol-blocked aromatic diisocyanates and thus may pose toxic and hazardous problems. Further, the liberated phenol may remain unreacted and produce a possibly corrosive phenolic environment in the fabric treater and other equipment.
Caprolactam-blocked diisocyanates, such as caprolactam-blocked 4,4′-MDI (e.g., GRILBOND® IL-6 from EMS-Primid), have been extensively used as ingredients in dipping formulations for isocyanate treatment of rubber reinforcing materials without a resorcinol-formaldehyde-latex (RFL); or as dip additives in other dipping formulations such as the single- and double-step RFL dipping formulations for treating rubber reinforcing materials. Like phenol-blocked 4,4′-MDI, the caprolactam-blocked 4,4′-MDI generally has a high unblocking temperature. In some instances, the adhesion of PET cords to rubber compounds may be enhanced by blending the phenol- and caprolactam-blocked 4,4′-MDIs together and using in RFL formulations.
In addition to phenol- and caprolactam-blocked diisocyanates, resorcinol-blocked diisocyanates such as 4,4′-MDI can be used in fabric dipping formulations. The resorcinol-blocked diisocyanates may provide some unique characteristics as an ingredient or additive in the dipping formulations. For example, the resorcinol liberated from the unblocking reaction of a resorcinol-blocked diisocyanate is more reactive than most other blocking agents, such as phenol or caprolactam. Therefore, resorcinol-blocked diisocyanate provides additional reactive resorcinol which is the major reactive component in the RFL-type formulations. Further, resorcinol-blocked diisocyanates have terminal phenolic hydroxyl groups which can promote the reaction between the resorcinol-blocked diisocyanates and epoxy compounds present in dipping formulations.
Although some of the problems associated with the use of phenol-blocked or caprolactam-blocked isocyanates in dipping formulations can be overcome by the use of resorcinol-blocked diisocyanates, all current blocked diisocyanates have the same characteristic of having only one unblocking temperature and/or one melting characteristic. However, in some high performance applications, it may be desirable to use a blocked diisocyanate having two or more unblocking temperatures and/or melting characteristics that may provide some unique properties, such as improved adhesion of various synthetic fiber materials to rubber compounds.